The present invention relates to a colour cathode ray tube, more particularly to a colour cathode ray tube in which optical filters are used to produce visible signals. The present invention is applicable to shadow mask tubes, particularly Datagraphic Display (DGD tubes), beam index tubes and other types of tubes in which previously a coloured image has been produced by elements which luminesce in different colours in response to electron beam impingement.
U.S. Pat. Nos. 4,634,926; 4,647,812 and 4,683,398 disclose a projection television apparatus in which multilayer optical interference filters are used to enhance the light output. In such an apparatus, which comprises three cathode ray tubes having screens luminescing in red, green and blue respectively, the phosphors are preferably selected to have line spectra, and the optical interference filter is disposed between the phosphor and the faceplate. The optical interference filter, which comprises a short wave pass filter, is formed by a plurality of layers manufactured alternately from a material having a high refractive index (H) and a material having a low refractive index (L). The filter has between 6 and 30 layers, and preferably between 14 and 22 layers, each having an optical thickness nd, wherein n is the refractive index of the material of the layer and d is the thickness, said optical thickness nd being between 0.2.lambda..sub.f and 0.3.lambda..sub.f, in which .lambda..sub.f is equal to p.times..lambda., where .lambda. is the desired central wavelength selected from the spectrum emitted by the luminescent material and p is a number between 1.18 and 1.36.
Striped optical filters are known per se. For example, an article "Striped Optical Filters Composed of Multi-Layered TiO.sub.2 and SiO.sub.2 Films Deposited By RF Sputtering" by Y. Shimomoto, Y. Imamura, A. Sasano and E. Maruyama Surface Science 86(1979) pp. 417 to 423, discloses making striped optical filters (cyan, magenta and yellow) for compact pick-up tubes using a multilayer RF sputtering apparatus. These filters are made of 13 layers of TiO.sub.2 and SiO.sub.2, with refractive indices (n) of 2.50 and 1.47, respectively, at 546.1 nm. The cyan (red) filter is (L.H).sup.6 L/2; the magenta (green) is (3H.L).sup.5 3H 1/2 and the yellow filter (blue) is (H/2, L, H/2).sup.6, where L and H respectively represent low and high refractive index layers with an optical thickness which equals 0.25.lambda..sub.o, where .lambda..sub.o is the centre wavelength of the filter. The filters are small, having a pitch, length and number of stripes of 20 .mu.m, 10 mm and about 700 lines, respectively. There is no suggestion of providing such optical filters on larger substrates.
Shadow mask colour cathode ray tubes for use in DGD applications have a number of requirements including: realising the smallest possible spot on the display screen to obtain a high resolving power; a high luminance in connection with a high contrast with respect to colour and location, and homogeneous controllability over the entire display screen, free from blending and flickering. Additionally, it is desirable for the decay time of the cathodoluminescent screen to be sufficiently short, and also for the X-ray emissions not to exceed the amounts legally stipulated.
The advantages alleged for a beam index colour cathode ray tube over a shadow mask tube are well documented and will not be listed here. However beam index tubes have a number of drawbacks such as, the necessity (so far) of providing extra black stripes and an ultra-violet phosphor between the red, green and blue phosphors; the necessity of making an electron beam spot smaller than the phosphor stripe width, that is an oval beam spot, which leads to a limitation of the beam current and thereby the brightness; the necessity of using impregnated cathodes, and the necessity of detecting ultra-violet light and of high-frequency correction as well as high-frequency switching (5 to 10 MHz) between red, green, blue and ultra-violet.
It is an object of the present invention to mitigate some of these problems in colour cathode ray tubes.